Termination resistance for reducing reflection of a signal is provided in an output terminal of a transmission circuit and a receiving terminal of a receiving circuit which are used for transmitting data. A value of the termination resistance is specified by standard or the like. The value of the termination resistance can be changed by operation conditions such as temperature. Accordingly, a device is proposed which corrects a value of termination resistance of a transmission circuit or the like so as to be included within a predetermined range (for example, Japanese Laid-open Patent Publication No. 2007-121288). For example, the device generates a control signal that adjusts output impedance of a device to be inspected, based on comparison results of an output voltage of the device to be inspected which duplicates a portion of the transmission circuit and a reference voltage. The control signal that adjusts the output impedance of the device to be inspected is also provided in the transmission circuit. Thereby, the output impedance of the transmission circuit is also adjusted.
As performance of a basic device or the like of an information processing device and a communication system of a server or the like has been increased recently, it is preferable to increase a data rate of transmission and reception of a signal in the inside and outside of a device. Multi-level transmission which uses amplitude modulation is known as a method of increasing the data rate per signal line. For example, a transmission circuit which uses 4-level pulse amplitude modulation (4PAM) includes a drive unit for a higher bit of data with two bits and a drive unit for a lower bit thereof. In addition, the transmission circuit performs amplitude modulation of the data with two bits, and transmits a signal (symbol) including any one of four voltage levels to a receiving circuit through one signal line. The receiving circuit detects a signal with two bits (for example, 00, 01, 10, or 11) from the one symbol. Accordingly, the transmission circuit which performs amplitude modulation of data with a plurality of bits and transmits the data through one signal line can increase a data rate per signal line, compared to a transmission circuit which transmits data with one bit through one signal line.
A difference between each voltage level which is output from the transmission circuit is specified by standard, and is designed to be included in a predetermined range. In a transmission circuit which uses 4PAM, for example, in a case where a value of termination resistance included in a drive unit for a lower bit is double of a value of termination resistance included in a drive unit for a higher bit thereby being stabilized, the differences between each voltage level which are output from the transmission circuit are equal to each other.
In a case where on-resistance or the like of a metal oxide semiconductor (MOS) transistor is used as termination resistance (termination resistance included in each drive unit) of the transmission circuit, an output voltage of the transmission circuit changes, and thereby a value of the termination resistance can change. That is, the value of the termination resistance of the transmission circuit can vary depending on patterns of input data to the transmission circuit.
Accordingly, although the value of the termination resistance at a specific voltage level (specific operation point) is adjusted to be included in a predetermined range, a ratio between a value of termination resistance included in the drive unit for a lower bit and a value of termination resistance included in the drive unit for a higher bit is not appropriately adjusted. The more ratio between the value of the termination resistance included in the drive unit for a lower bit and the value of the termination resistance included in the drive unit for a higher bit deviates from an appropriate value, the more variation of the difference between each voltage level of the voltages which are output from the transmission circuit increases, and characteristics of a transmission waveform is degraded.
According to one aspect, a termination resistance adjustment circuit and a device including the termination resistance adjustment circuit of the present disclosure aims to appropriately adjust the value of the termination resistance of the transmission circuit.